For a while now my car has been complaining about P0128 and P2128. Both are related to engine temperatures and irregular thermostat I know that, but in the mornings when it is cold (below 45 degrees) I'll sit at idle for a few minutes to let the engine warm up before I commute to work. Generally the temp gets up to 150 degrees and the defrost mostly works but after driving for 10 minutes my engine temps will continue to fall (cruise set to 70 the whole way on the freeway) until finally I'll get the ETC light and my engine temp will just drop completely to 32 degrees and not move at all no matter what I do. After arriving at work turning the car off and then driving home it's reset and the car functions like normal (functions normally even when the ETC light comes on). I have also noticed that when this happens the radiator fans turn on full blast no matter what. I am curious what could be causing this? Here is a photo of what I am talking about. Thanks for reading

It could be the ECT sensor,also check your coolant level,and make sure your coolant is set for -34 or better.

Coolant level is good and is the correct temp rating. If it is the ECT sensor what would make the dummy light turn on and my reported engine temp drop from 100ish to 32 and not move? I'm not concerned about this considering it's complaining that my engine temp is too low
Any other ideas? I'm going to disconnect my battery this weekend and let the car sit all day to clear the codes and see if it happens again.

What does your temperature gauge do when this happens? The ETC light (Electronic Throttle Control) has nothing to do with the coolant temperature but something is triggering both events. Is the heater air as hot as it normally is or does the air temp drop during your commute? If so, start with replacing the engine thermostat.

What does your temperature gauge do when this happens? The ETC light (Electronic Throttle Control) has nothing to do with the coolant temperature but something is triggering both events. Is the heater air as hot as it normally is or does the air temp drop during your commute? If so, start with replacing the engine thermostat.

As I am driving the heater gets cooler and cooler till it blows just cold air. The temp gauge will gradually decrease till it hits the end point. After that happens is when the ETC light comes on and throws a P2128. I am still curious as to why the radiator fans turn on full blast as soon as my ETC light comes on. I will replace the thermostat. I'm sure that will resolve the problem, thanks for your response

For a while now my car has been complaining about P0128 and P2128. Both are related to engine temperatures and irregular thermostat I know that, but in the mornings when it is cold (below 45 degrees) I'll sit at idle for a few minutes to let the engine warm up before I commute to work. Generally the temp gets up to 150 degrees and the defrost mostly works but after driving for 10 minutes my engine temps will continue to fall (cruise set to 70 the whole way on the freeway) until finally I'll get the ETC light and my engine temp will just drop completely to 32 degrees and not move at all no matter what I do. After arriving at work turning the car off and then driving home it's reset and the car functions like normal (functions normally even when the ETC light comes on). I have also noticed that when this happens the radiator fans turn on full blast no matter what. I am curious what could be causing this? Here is a photo of what I am talking about. Thanks for reading

You didnt mention what year or engine you have,but I came across 2 TSBs with the same Codes you have,the Po128 is for the 3.5 engine,08/10 and the P2181for all LX 07/10,here are the TSBs: